For example, a device transferring high-pressure fluid such as a fuel injection device conventionally has a sealing structure for sealing an annular gap formed between a housing having a shaft hole and a shaft inserted into the shaft hole. For this sealing structure, a seal member (an o-ring or general rubber-like elastomer material such as a u-packing) is mounted along with a backup ring made of a resin material such as PTFE (polytetrafluoroethylene) coaxially disposed in parallel with the seal member on the pressurized side opposite to the seal member.
The sealing structure is solely achieved by the seal member and the backup ring functions to prevent the seal member from protruding to an annular gap between an outer circumferential surface of the shaft and an inner circumferential surface of the housing due to pressure of hydraulic oil etc.
The seal member is made of a rubber-like elastomer material and therefore can be stretched to a larger diameter at the time of mounting on the outer circumference of the shaft. However, the backup ring is made of a synthetic resin material harder than the seal member and, therefore, a cut portion 103 is formed by cutting at least one circumferential position as depicted in FIG. 8 so that the backup ring can be opened to a diameter larger than the outer circumferential surface of a shaft 200 (see FIG. 9) in consideration of mountability.
A technique described in Patent Document 1 depicted in FIG. 9 will be described as an example of the backup ring. Elastic deformation of a backup ring 100 mounted on an outer circumference of a shaft 200 is utilized not only to eliminate a gap between the shaft 200 and the backup ring 100 but also to allow the backup ring 100 to slide relative to the shaft 200, thereby preventing the formation of the gap.
Therefore, in this conventional example, a sloped surface 201 is disposed on a low pressure side (L) of a groove bottom surface of an annular gap 400 disposed on a shaft-hole inner circumferential surface of a housing 300, from a high pressure side (H) toward the low pressure side (L) (see FIG. 10). An entire inner circumferential surface of the backup ring 100 disposed on the low pressure side (L) adjacently to a seal member is made up of a taper surface 102 sloped along the sloped surface 201 from the high pressure side (H) toward the low pressure side (L). The angle of the sloped surface 201 of the annular gap 400 and the angle of the taper surface 102 of the backup ring 100 are made equal such that sealing is achieved between the both surfaces.